Every year I go through my early season dis-belief at how much my FC goes down every day. Right now it seems to go from 6 to 2 (or lower) every full sun day. The house is on the east side of the pool and large trees on west but for at least 8 hours the sun hits pool directly. I am now onto a new batch of chlorine, 10% from Home Depot, $9/ 3 gallons. Seems like its holding out a tiny bit better than slightly older 12.5% from Leslie's ($20/4 gallons) was. I store chlorine in hot garage, which may speed deterioration? Have fun out there in pool land! Much thanks to TFP for putting me on the right track.